What type of unintentional killing is called wicked?
(21,13) “But if he did not premeditate it, and G-d arranged it to happen through him, I will make a place for you to which he shall flee.”
There are two types of non-premeditated killing. The first is where one could have taken a precaution, for example, where someone was chopping wood and a splinter of the wood flew off and killed a person - he should have made sure no one was standing there. The second type is where one could not have taken a precaution, for example, someone was climbing a ladder and one of the rungs broke from under him, causing him to fall on a person and kill him.
Both these types are mentioned in our posuk. When it says “but if he did not premeditate it” it is referring to an unintentional killing that he could have guarded against, and “G-d arranged it to happen through him”, refers to an unintentional killing that he could not have guarded against.
And the reason why Hashem arranged this unintentional killing to occur is as Dovid Hamelech said in Shmuel Ⅰ 24:7 “as the proverb of the ancient one (the Torah of Hashem) says: From the wicked shall go forth wickedness”. That is, because “he did not premeditate it” but he also did not guard against it, he is called wicked, and since there were no witnesses to the event he is not required to go into exile. Therefore, “G-d arranged it to happen through him” - He arranged that he will fall from a ladder and unintentionally kill a wicked person who killed intentionally but without witnesses, as the gemora in Makkos 10b explains.